Propanil has been used widely to control or combat the undesirable weeds in the fields of rice plant, especially in the irrigated paddy field of rice plant for many years. Propanil are usually formulated into the forms of wettable powder and emulsifiable concentrated solutions containing propanil which may be mixed and diluted with water before its use and then sprayed for the herbicidal purpose. From experiences in the past, however, it is said that the spraying aqueous emulsions as prepared by dilution of the emulsifiable concentrated organic solutions of propanil with water can ordinarily give better herbicidal effects than those obtainable by spraying the aqueous suspension of propanil as prepared by dilution of the wettable powder of propanil with water, when comparisons are made at the same rate of application of propanil to weeds. Accordingly, the emulsifiable concentrated solutions of propanil have higher practical utilities as the herbicidal agent and have been preferred than the wettable powder of propanil.
Thus, the herbicidal effects obtainable by application of propanil are different, in fact, depending on the difference in the forms of the formulation containing propanil. This difference in the herbicidal effects exerted by different formulations of propanil employed is said to be attributable to the difference in the physical state of propanil which exists in the spraying aqueous emulsion or aqueous suspension of propanil as formed by dilution of the original emulsifiable solution or wettable powder of propanil with water. Namely, the above-mentioned difference in the herbicidal effects obtained by different formulations of propanil employed may be attributable to whether the propanil present in the spraying aqueous emulsion or aqueous suspension after dilution with water is precipitated in crystalline form or amorphous form or is retaining the form of a solution of propanil to a substantial extent. Besides, even when an emulsifiable concentrated solution of propanil in organic solvent(s) is employed and diluted with water to prepare a spraying aqueous emulsion of propanil, the herbicidal effects obtainable by application of said spraying aqueous emulsion cannot be sufficient to achieve the herbicidal purpose, if the emulsified state of the propanil solution as the disperse phase has been destroyed in the resulting aqueous emulsion after dilution with water, resulting in that propanil can precipitate as crystals or as solids form from the water-diluted emulsion as formed. For these reasons, in principle, it is essentially required that an emulsifiable concentrated solution of propanil which is well practicable should show such properties that the fine liquid particles of said concentrated solution resulted as the disperse phase within the aqueous emulsion formed after the dilution with water are stably remaining dispersed in the continuous medium of water without involving precipitation of propanil, either as crystalline or amorphous solids, from the water-diluted emulsion. That is to say, it is required that the well practicable, emulsifiable concentrated solution of propanil should show such nature that this emulsifiable solution of propanil is well dispersible in water and is stable in the sense of having no or little tendency to precipitate or crystallize its propanil component as solid from the aqueous emulsion as formed upon dilution of said emulsifiable concentrated solution of propanil with water. This nature of the emulsifiable concentrated solution of propanil that the latter solution of propanil is dispersible in water and can lead to a stable condition of the emulsion or the stable disperse organic phase in the water medium after the dilution with water but does not involk precipitation of propanil solids is hereinafter referred to as "emulsion stability in water" or merely as "emulsion stability".
Besides, of course, the well practicable, emulsifiable concentrated solution of propanil itself should be highly stable in the sense that it has no or little tendency to precipitate or crystallize its propanil component as solids from said concentrated solution of propanil upon its storage during which it is stored for a long time at room temperature or at lowered temperatures. The stability of the emulsifiable concentrated solution of propanil in the sense just mentioned above is hereinafter referred to as "storage stability".
Furthermore, propanil itself has low solubility in hydrophobic or water-immiscible organic solvents such as aromatic solvents which are conventionally employed in usual, emulsifiable concentrated solutions of certain pesticides. When these water-immiscible organic solvents are used, it is naturally very difficult to prepare an emulsifiable concentrated solution having a high content of propanil. On the other hand, propanil has high solubility in hydrophilic or water-miscible organic solvents such as acetone and cyclohexanone. Therefore, it is easy to prepare an emulsifiable concentrated solution having a high content of propanil when the water-miscible organic solvent is used to dissolve propanil. However, in case when such emulsifiable concentrated solution of propanil in the water-miscible organic solvent is diluted with water, the "emulsion" state of the resulting water-diluted emulsion that the fine liquid particles of the organic solution of propanil are remaining as the dispersed phase in the continuous water medium can be destroyed in short time, as the fine liquid particles of the organic solution of propanil dissolve into the water medium relatively quickly. It is very difficult to prepare any well practicable, emulsifiable concentrated solution of propanil with using the water-miscible organic solvents in which propanil is highly soluble.
Thus, the above-mentioned difficult problems involved in preparation of the stable, emulsifiable concentrated solution of propanil have heretofore been attended by using isophorone, namely 3,5,5-trimethyl-2-cyclohexen-1-one as the organic solvent for dissolution of propanil.
However, as is described in the literature "Inert Ingredients in Pesticide Products: Policy Statement" (Federal Register/Vol. 52, No. 77), isophorone is objectionably toxic as this solvent is included in the class of compounds having strongest toxicity classified by E.P.A. (Environmental Protection Agency). Hence, commercially available emulsifiable concentrated solution of propanil in isophorone can incur the problem of toxicity stemming from isophorone.
Preparation of emulsifiable concentrated solution of propanil without use of isophorone may be achieved by use of cyclohexanone or 4-methylcyclohexanone as a solvent with aid of triethanolamine dodecylbenzenesulfonate as an anionic surfactant along with a nonionic surfactant and in combination with an aromatic solvent (Japanese Patent Application first publication "Kokai" No. 176902/82). Additional incorporation of a polyaromatic compound containing 2 to 5 aromatic rings, for example, dibenzyltoluene has also been proposed (Japanese Patent Application first publicaiton "Kokai" No. 9901/89).
Emulsifiable concentrated solutions of propanil which are prepared using the above-mentioned cyclohexanone, optionally together the organic co-solvents, however, can involve the drawback that solids or crystals of propanil precipitate when such emulsifiable preparations of propanil are diluted in water at 15.degree. C. or lower, followed by vigorous stirring and then allowing to stand. Commercial emulsifiable concentrated solutions of propanil in isophorone used as the principal solvent are also accompanied by the drawback that solids or crystals of propanil can often precipitate upon dilution with water.
Emulsifiable concentrated solutions of propanil are usually used in a relatively early stage of spring for the herbicidal purpose, and the temperature of water employed for the dilution is generally low. Further, when the aqueous emulsions as prepared by dilution of the emulsifiable concentrated solutions of propanil with cold water are sprayed, the water-diluted emulsions so prepared need to be repeatedly stirred and allowed to stand in a chemical tank and then, propanil tends to precipitate as solids or crystals from the aqueous emulsions so prepared. The spraying application of such aqueous emulsions containing propanil solids or crystals precipitated, therefore, can lead to insufficient herbicidal effects and also be accompanied by some troubles upon spraying them due to that spray nozzles can clogg by the precipitated propanil solids or crystals.
Other methods for preparation of emulsifiable concentrated solutions of propanil include the use of a ketone, such as methyl n-hexyl ketone, as the solvent for dissolution of propanil (Japanese Patent Application second publication "Kokoku" No. 9275/67). The emulsifiable concentrated solutions of propanil prepared using the ketones are free from the trouble of precipitation of propanil solids or crystals from the water-diluted emulsion and have good "emulsion stability" but can be accompanied by another problemes that, when stored at low temperatures over a long time, solids or crystals of propanil precipitate from its solution and cannot be easily re-dissolved even when allowing to stand at room temperature for a long time.